There are levels in college tennis just like anywhere else. A former top college player said the difference between the pros and college is that in college tennis you know there are only a couple of guys who you know can beat you if they're playing their best, whereas in the pros, you know that EVERYONE can beat you if they're playing their best. In other words, there are at best only a few true pro caliber players in college tennis. The rest are middling by comparison in talent, drive, speed, athleticism, power, touch, court sense, mental toughness, or all of the above.
Remember, Gene Mayer at 40 something was on vacation in Hawaii with his family when asked by an old buddy to fill in at the last second at a challenger tournament. Mayer had not played seriously for years, and was not in match shape at all. Yet, he still SMOKED...absolutely smoked...Cedric Kaufman who only a few short months later would give Pete Sampras everything he could handle at the French Open. In the next round, Mayer was said to be schooling Mike Bryan (a former standout collegiate player not too far removed) as well...before his lack of conditioning caught up to him. He basically said afterward that he decided to just let him win at that point.
So to answer the question, can a former pro beat a current college tennis player? You better believe it. John Isner remember lost to Kevin Kim not that long ago, and Kevin Kim could definitely lose to a former top pro. I think people overestimate the difference in levels in tennis. In my opinion, once you get to top 300 level or so, there really isn't that much difference...it's all subtle at that point, and that's why you can see someone like Isner or Nishikori make such a splash one week and then come back down to earth another without seemingly any rhyme or reason.