Is it ever ok to try on someone else's pointe shoes?

In general, the answer is no.  However, I recently heard a story from one of my friends involving Ballet West's Allison DeBona trying on a student's shoe at a summer intensive where she was teaching.  Now I feel the need to make a few things clear.

If you are a non-dancer, it is never, ever, ever ok to try on pointe shoes, regardless of whose they are.  You wouldn't jump in a race car and start speeding around the track if you've never even taken driver's education classes.  It's the same with pointe shoes.  It's unreasonable to expect that you will be safe trying something advanced if you haven't learned the basics.  Dancers train their bodies to perform movements and hold positions that are not innate, so even if you're relatively strong or athletic, you probably won't have the specific foot and ankle strength needed to rise onto pointe safely.   (Note that there is nothing wrong with lacking this skill-it doesn't make you "weak" or take away from your unique talents and abilities.)  If you are interested in your friend's pointe shoes, it's absolutely ok to ask to touch them or for her to tell you about her pointe experiences.

If you are a dancer who is not (or not yet) practicing pointe, do not try on pointe shoes (unless, of course, you are having your first pointe fitting).  If you do a style of dance like jazz or tap, you are not training the same muscles as ballet dancers.  While your dance style keeps you strong and healthy, it probably does not prepare you for the specific physical requirements to safely dance en pointe.  If you do ballet and your teacher has not yet promoted you to a pointe class, you're still working.  After some time (amount depends on age, years dancing, etc.), you will be strong enough to earn your shoes.  Stepping into pointe shoes now can cause you to get injured, which will increase the amount of time you must wait until you're ready.

If you are a beginning pointe dancer, you should not try on someone else's pointe shoes.  You definitely have the strength and flexibility to start learning pointe, but there's much more to it than ability.  When you were fit for pointe shoes, the fitter and/or your teacher took time to analyze your feet and choose shoes for you that would best meet your needs.  Even if you tried on multiple pairs, each one was selected because it had potential to work for you.  If you try on a friend's shoe, it may be too big or too small or too hard, and each of these elements can cause you to fall and hurt yourself even if you are being careful.

If you are an advanced pointe dancer, you should still avoid trying on other people's shoes.  The reasons for beginning pointe dancers still apply, though you're most likely a lot stronger and in control of your feet than you were when you first started pointe.  Every person breaks in shoes differently, and rising onto pointe with the arch and demi in a different place can be disorienting enough to make you fall.  Also, pointe shoes can breed bacteria, especially if they are generally worn without tights and/or toe pads.  Putting your feet in someone else's shoes is kind of like putting on someone else's sweaty socks.  It can spread fungus like athlete's foot if you're not careful.  It's better to stick to trying on new pointe shoes at the dancewear store if you need a change.

If you are a professional (i.e., no longer a student), and/or in an advanced enough level that you manage your own pointe shoe choices (your teacher does not approve the shoes, you choose your own toe pads, etc.), it's still not the greatest idea to try on someone else's shoes for the health and safety reasons listed above.  However, by this point in your training, you've definitely amassed a large body of knowledge about pointe, pointe shoes, and what specifically works for you.  There's one situation where it might be ok to give someone else's shoes a go (and this is the case with the Allison DeBona story):  If their shoes are (relatively) new and there's a reasonable chance the size and model will work for you.

Most pros have as much knowledge (or more) about pointe shoes than fitters and teachers, and they will purchase or request shoes with minor adjustments (up a width, etc.) without going in for a fitting or asking a teacher first.  They develop the ability to imagine adjustments to their shoes and how they will affect the feet, like thinking, "I bet I'd be able to balance a bit better and have less pain in my bunion if my box was a tad wider."  If that dancer was in class and saw a friend putting on a pair of brand new pointe shoes that look very much like her own shoes, but with a slightly wider box, it may be reasonable to ask to try them on and see if the imagined adjustment is actually beneficial.  In this case, tying on a friend's shoes eliminates the expense of buying (or having the company buy) the shoe without knowing if it will work.

Of course, if a dancer tries on a friend's shoe, she must be very careful.  She should first evaluate the shoes on her feet without rising onto pointe.  Once that seems ok, she should very carefully rise up while holding the barre.  Care should be taken with the shank so she doesn't break them in the wrong place for the dancer who owns them.  If the "fitting" went well, the dancer should take note of the shoe's brand, style, and size and consider ordering some for herself.

Overall, remember that it's almost always a poor decision to try on someone else's shoes.  Unless you're a seasoned professional or very advanced dancer with lots of experience and autonomy, it's safest and smartest to stick with your own shoes that were fit for you or refrain from trying pointe shoes all together.