Can I take glasses from someone lax kosher-style home and make them kosher?
Regarding kashering glass that was used for non kosher foods, the Sefardi minhag is to be lenient and washing them well will suffice. According to the Ashkenazim however, there are different opinions, most poskim say that although we are strict regarding Pesach, however if it was used for non kosher foods it may be kashered.
Note: The Ashkenazi minhag is not to kasher glass for Pesach.
If the glass only absorbed from cold, i.e. it has cold no kosher food in it for 24 hr., then there is an option to fill the item until the top with water, and leave the water in there for 24 hours, then the water is spilled out. This process is done three times, and it will kasher the glass. However in your case it was used for both cold and hot.
O:CH 451-26, Y:D 135-8, Rema O:CH 451-26, R’ Shlomo Kluger ( third edition 2-25), Bais Lechem Yehuda 121, Chamudei Daniel, Basar Bechalav 45, Minchas Yitzchok 1-86.