As a “YU Bochur” myself I take offense to the doubt that you have of allowing your daughter to go out with this “good guy”. Many talmidim in my yeshiva are just like the talmidim that learn in any “yeshivish” yeshiva and go to college, many are even stronger than some who learn in a “yeshivish” yeshiva. They learn strong for two sedarim (maybe even three), are serious about their avodas Hashem, have good middos etc. etc. the only difference is that as opposed to driving from yeshiva to college, we walk across the street to college. True, there are a wide range of talmidim in YU, some more modern than others, but I assume that your daughter is not being redt a truly “modern” guy. The guy that is being suggested for your daughter is probably very similar to the guys she has already gone out with.
“but he went to one of the Hesder Yeshiva’s in Eretz Yisroel and might have shittos that are a little bit more modern”
This can happen with any guy, even with one who learns in Lakewood, Mir etc. everyone has a different Rebbe so it is inevitable that certain shittos will different (aka “modern”). There is a reason a guy and a girl go on dates. To see if they are compatible. Nobody is suggesting that your daughter marry this guy without going out with him. There are certain things that can not be learnt about someone without meeting and getting to speak to them. In the end it may not work out with this guy, but she may see that there is something specific about YU guys that she likes that she would not have known without going out with him.