If an individual holds from Cholov Yisroel , may they eat in someone's home that uses cholov stam . In particular may they eat on dishes that have been used for cholov Stam?
In the Kosher Kitchen page 317 Rabbi Forst writes that the common custom is to be lenient.
However, many other poskim forbid it and that is the Minhag in many communities such as Chabad.
You can also find more on this in this stack exchange question.
Rashba explains that in general when Chazal create an issur, k’ain d’oreisa tiknu (the gezeira is modeled after an issur Torah). Therefore, gevinas akum and chalav akum will assur b’taaruvos, unless it is batel b’shishim, and likewise they will assur keilim. Shach (Y.D. 115:17) cites the opinion of Issur V’Heter that indeed gevinas akum is batel b’rov, which would presumably be based on comparing gevinas akum to any other issur shehu’amad, but Shach rejects this position. Chochmos Adam 67:5 writes regarding gevinas akum, that even b’hefsed mirubah, one should not rely on Issur V’Heter and gevinas akum should be treated like every other issur. However, regarding chalav akum, which is more kal than gevinas akum, Chochmas Adam writes that if it is a hefsed mirubah one may rely on Issur V’heter that chalav akum is batel b’rov. Based on this Chochmos Adam, we can perhaps answer a perplexing question. What is the reason why some people who are makpid to only drink chalav Yisroel are not makpid when it comes to using keilim of chalav stam. We pasken ta’am k’ikar, so what rationale can there be to be machmir on the ikar and yet maikel on the ta’am? But according to Chochmos Adam, we see that b’makom hefsed mirubah, chalav akum is batel b’rov. This means that regarding chalav akum, perhaps Chazal did not pasken ta’am k’ikar. Since these people b’ikar pasken like Igeros Moshe that chalav HaCompanies is permitted, and have only accepted chalav Yisroel as a chumra, regarding the keilim, where there is more room to be maikel, they have not accepted this chumra.