Agreed that "the first girl" Lindsay is talking about is Tina, who was the first girl Angel helped as an agent of The Powers That Be. And it's not really a stretch to think that Lindsay could have knowledge about this, since Russell Winters(who killed Tina) was Lindsay's client.
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Superstar agrees: So he knew it was Angel's "first" exactly how?
He may or may not have known for certain, since he said "I hear Angel let the first girl die." Granted, they couldn't be sure how long Angel was in L.A., but this was the first event that put him on W&H's radar - so this is probably what they reference as the beginning of Angel's timeline in Los Angeles. With all the resourse that W&H put into learning everything they could about Angel(Angel's cabinets upon cabinets of history in W&H's Files & Records as mentioned in Season 3, for example), W&H and Lindsay may well have been pretty certain that this was 'the first girl'.
Again, none of this was ever explained on the show, but just my own assumptions from watching.