I (30F) found out that my boyfriend (32M) has been 'helping' his female friend (30F) get her life back together, and that he never told me because he thought I would think he is sleeping with her.

I overheard a conversation my boyfriend was having with someone, and I specifically remember him saying he hadn't told me about 'what we're doing'. I right away felt my stomach drop, like what? I do remember going on his computer to check my email and seeing he had some Facebook messages open with a girl, but I didn't want to snoop so I never looked into it much. I figured it was a coworker. I decided I had enough suspicious reason to check those messages. I went and saw them and at first I was worried, but after reading them I was relieved, and VERY confused. This girl, Andrea, seems to be a heroin addict who came out of recovery but was homeless for a while. I have no idea how he knows her, but he has been helping her with all kinds of stuff, both just talking to her and mental support, and also like, helping her find an apartment and helping her move and buying her groceries. I was just shocked. It wasn't a lot, at all. This was all over the span of a year and it seems they talked like once every month or two. He wasn't running over to help her install a microwave every week. She has some boyfriend apparently who is also an ex addict but has had a TBI and has a lot of problems related to that. Judging from the conversations, I could tell he wasn't cheating with her. I'm like 95% sure.
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I confronted him about it, I wasn't mad at all, but I was very very confused as to why he wouldn't tell me about her. Some part of me knew the answer but was a bit afraid of it, and he told me exactly what I was afraid of, that he thought I would be suspicious of him sleeping with her. He said he lied about the first few times he helped her out and every time after that was him avoiding the fact that he was already lying. I just told him that he should have told me, and that him lying just makes the whole thing more suspicious, and he said he knows, and that is why he didn't want to tell me, and he almost hoped that it would just end. I didn't really know what to say. I didn't realize he didn't trust me like that. I get how lies, even simple ones, can spiral into more lies and become out of control, but the important thing is that you stop lying before it gets to that point. I told him that, and he agreed and said he felt terrible. He said he was friends with her in middle school and high school and lived down the block from each other, and they sort of remained friends after.
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Not really sure how to feel about this. How am i supposed to feel about something like this? We've been dating for 4 years now.
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TL;DR - - boyfriend lied to me about helping his friend (girl) because he thought that I would assume he is cheating


Source.