What is your advice to me regarding my friend whose behavior towards me has changed after he met someone else?

One should investigate the reason for a friend's estrangement and reconcile with them. If that is not effective, there is no harm in treating them in kind. Also, one must be moderate in estimating and loving friends, for friendship may cease to exist, as the Prophet, peace and blessings be upon him, said: "Love your beloved moderately, for he may become your enemy one day; and hate your enemy moderately, for he may become your beloved one day."