I went to a distributor's showcase a few months ago, where they sampled out a lot of their wine, beer & spirits from their portfolio and tried 2 great (to me) Port wines.
Look for:
Dow's 2011 Late Bottled Vintage Porto
and
Dow's Quinta Senhora da Ribeira - Vintage Porto-
I really liked the 2011 bottle the best. Not sure on pricing. If these are not avail or too pricey for your mom, tell her to look for something at least 5-6 years old or older. The older it is, the more it cost. Just remember, a Port wine is more of a dessert wine, so you normally don't drink a lot. If you go to a nice restaurant and order a Port, it normally comes in a small wine glass, unlike a larger normal wine glass.
These 2 have been a decent drink also, which should be under $20 a bottle:
Fonseca Bin No. 27 Port Wine. ...
Graham's Six Grapes Reserve Port.