Any pork other than Greek or spiedes, I'd say hold the lemon until the end. But that would go to controlling the lemon taste and not from fear of acid.
Generally, there's no problem with some acid in a marinade as long as it's not overwhelming. Indeed, by definition, all marinades contain some acid. Acids power the process of diffusion which carries the flavor of the marinade into meat. Wines, vinegars, and citrus are typical acid sources in marinades.
There's no technical problem with lemon in a pork marinade -- especially for a piece of meat large enough to term a roast. Pork is not structured like chicken (especially pieces), so in this case, the advice to avoid lemon juice is too probably overcautious.
The question is -- how much lemon were you thinking of putting in. It's possible to overdo it. Certainly too much acid of any sort for too long a time will make meat mushy. But two or three lemons in a lot of olive oil, some wine, along with plenty of garlic, for an overnight sleep in the refrigerator shouldn't be any problem at all. Don't forget the rosemary.
Squeezing lemon over pork on a rotisserie for the last hour or so, is pretty much a waste of time. Lemon aroma and taste is evanescent at heat, and will simply cook off. Hold off until the last five or ten minutes, and you'll get everything you're going to get. It's common in Greece and the Eastern Med to crumble dried oregano over the meat during the last few minutes -- alont with the lemon squeeze. Of course, the flavor you get this way will be concentrated on the surface of the roast -- think of it as an adjunct to the marinade.
To answer Mapiva's question -- at first the acid will tend to "seize" the meat; and it will contract and toughen. However as the acid works on the meat protein molecules and begins to denature them the meat gets tender and may even get mushy. So, you're mom's right even though so is everyone else. How do moms rig it that way?