Is accepting marriage to a friend's husband, who courted the questioner after a failed marriage attempt with another party, considered a betrayal of friendship, taking into account the questioner's age and limited marriage opportunities?

If a man intends to marry and is sincere in his intention, there is no impediment to that, provided the marriage is completed according to Islamic law with its known pillars of contract. Your conversation with him should be limited to what clarifies his intention, while being careful not to let Satan lead you to what displeases Allah.