Why would a guy never mention or talk about an old friend (female) to another girl while he tell her about other girls?

So i know this guy that had an online friendship lets say with benefits with this girl but he never mention her or talk about her to a new girl he meets even though he talk about his other female friends and exes, even when a girl asked if he has friends from that country, he said no. What could this means?

Why would a guy never mention or talk about an old friend (female) to another girl while he tell her about other girls?
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