HP5+, possibly expired

So a friend gave me a whole bunch of film, different brands, speeds, types etc. There's a roll of HP5+ in there (w/out packaging) that he says could be anywhere between 5-20 years old. So my question is, should the film be expired, what results do I have to expect, and is there anything that could go wrong during the processing?

If it's a single 36exp roll, it's $4 or so new. But go ahead and shoot it on something which isn't that critical. What happens first is that the so called "base and fog" builds up. The negatives get somewhat darker compared to the negatives from fresh film, but they are still useable. If the film is too old, the results will be useless.
Last, apart from the negatives being anything from a bit low in contrast to being useless, there is nothing else that will happen with old film.

//Björn

Alright, thanks for helping!