Even if he's a friend, you might want to draw up 
a contract specifying the amount of time the cels 
will be up, how they will be displayed, how they 
will be treated, any security precautions that 
will be taken and who will pay for them, how they 
will be returned for you, whether or not any 
reimbursement will be made for damage incurred 
while on display, and so on.

If you do lend them out, take good scans of 
anything you send away and look over them 
carefully for any signs of damage. Draw up a list 
of everything you loan.

That's from the museum point of view. :o) If 
objects aren't properly documented upon leaving, 
and if everything isn't spelled out beforehand, 
it can sometimes be a real pain to get stuff 
back. It helps if you're close by, but still, no 
one likes not knowing whether or not they can get 
their stuff back. :o) When I was working in one 
museum, I came across something that we had 
borrowed from somewhere else about forty years 
ago. Of course, dead frogs in jars and cels are 
two different things... and you're more likely to 
not wait forty years to get something back... but 
documentation is always important, as is a 
legally binding contract, just-to-make-sure for a 
worst-case scenario.

The second item to take into account is where 
they're going to be displayed. You say it's a 
bar/restaraunt. Is there smoking allowed indoors? 
Is it brightly lit, or will your cels be exposed 
to any areas that are constantly lit? (Cumulative 
damage from a minor light source is just as bad 
as damage from a major light source.) Does the 
air feel greasy or have that "food-y" kind of 
feel to it? All of those may affect your cels. 
The atmosphere is going to affect your cels if 
they aren't sealed within their frames, and the 
light's going to damage it unless you've got some 
sort of UV filter to diminish any affect. You 
don't want someone hanging it right next to a 
lamp that's always on, or in front of a window. 
You don't want your cels displayed in the smoking 
section. That kind of thing. Also, be careful--- 
some places have big problems with 
decorations "walking off", whether it's flower 
arrangements on the table, or honkin' big 
decorative platters hanging on the wall. 

If you're comfortable with the situation, by all 
means, go with it. Just be sure to take 
everything into account before you do so, and if 
you decide to go forward, take basic precautions 
to protect yourself and your collection.

Good luck,
-Cres