Sprint will allow him to switch to whatever he wants... as long as he pays for it
... the TP->TP2 swap is the only one that's happening under TEP/Warranty. If your friend wants to switch from a BB to a Pre, he'll likely have to pay. If he's had his BB for 2 years, he's most likely eligible for the upgrade discount. If not he'll have to pay full price.
Edit: Just remembered one other possibility. Supposedly, if you've had multiple exchanges of the same type of phone within a certain number of months (and I've heard varying figures, like 3 exchanges in 3 months, 3 in 6 months and a few others), they're supposed to offer a different, but comparable, phone at that point as well under TEP.