There could be a very simple explanation for this. I would not jump to any conclusions.

leave them alone

Ask the explanation to him carefully,when we spend time together ... Maybe,when i saw it i will jealous ... But its normally for that situation ...

Well, I would feel jealousy first and pretend haven't seen them, then I would ask him who is the girl when we get together.

i think its better ask her/him and not to think that they have relationship,cause it could be friends or relatives.theres no harm in asking. cowboy...