I would assume that most people who are makpid on chalav yisrael avoid eating milchiks at people’s houses who aren’t makpid on chalav yisrael. This shouldn’t be a big deal most of the time, especially since most of the time people eat at other people’s houses on Shabbos and Y”T when they aren’t serving milchiks.
In the above situation, either Shimon can buy c”y milk for Reuven, or Reuven can provide his own milk. Reuven and Shimon are clearly good friends if Reuven eats at Shimon’s house during the week so often, so it shouldn’t be a big deal for them to discuss it and work it out in one of the above two ways.