Is it permissible to remind a girl who was in a transient relationship about religion, and inform her that what she is doing is wrong so that she avoids it, or not, knowing that the questioner avoids speaking with them except out of extreme necessity?

Frequent mixing with girls is a cause for error. The basic principle is to prohibit conversing with a non-mahram girl unless there is a need that cannot be fulfilled by young men, and with the assurance of no temptation and the absence of lust, and limiting it to the extent of the need. Enjoining good and forbidding evil is included in the need that permits conversation. It is preferable that one of the sisters advise her, or that an indirect method be used.