Pictures of the barrel flats and the water table would help lots. Pictures will be required for any attempt at valuation, anyways, because even an XE can be worthless if it's beat up. I realise that you mentioned it's in good shape, but that can mean very different things to different people. The precise condition is very important to a valuation such as you're seeking.
The good news is that it is almost certainly worth more than your average old shotgun. Fox shotguns fall into 3 major categories with umpteen sub-categories for each. The main categories I speak of (my distinction - not official "categories" accepted by all collectors):
1) Graded
2) Sterlingworth
3) Model 'B' (NOT to be confused with GRADE B)
Starting from the latter and working up:
The (Savage)Fox model B is not a Fox gun at all, but a higher end Stevens model 311 SXS. My personal opinion about a Fox model B is that it is well regulated and shoots well, but suffers from design issues that compromise dependability (ask me about my model B and you'll get quite a story).
The Sterlingworths, on the other hand, are a cost-trimmed, no frills, field model that share all of the design advantages of the graded Foxes, but utilize the less expensive material alternatives and finish options available from the Fox factory at the time. As much as possible, the idea was to keep the price as low as possible while retaining the simple, reliable Fox action design. In this, they were largely successful.
The graded guns start at 'A' Grade and are progressively more elaborate through B, C, D, and a variety of other grades all the way up to XE, Presentation, and Exhibition Grade. Available grades varied from era to era and there are examples of skeet, trap, and other versions that are considered by many to be "configurations" of existing grades, as opposed to separate grades.
Both the Sterlingworths and the graded Foxes have a loyal following.
Hopefully you can post some images. It'll be fun trying to nail down what your friend's father has (BTW, if he has any info about what grade he thinks it is, it might make a good starting point.).