Is this a good reason?

So im a girl dating a guy. He USED to be into drugs and alcohol, but stopped about 10 years ago. Recently his mom had to go into the hospital, she's dying. I normally stay the night at his house with him but i couldnt for two days. And in those 2 days he asked another woman over to his house so he could get some pills off her. He deleted the messages and asked his sister not to tell me. Well the woman wrote on a piece of paper I had made for him about a week ago, and it had her name with a bunch of hearts. It was her handwriting not his. So I texted him about it, and asked when the last time he saw her. I also said I have the evidence I showed him the evidence and I waited for his response. I am very religious, and do not want anything to do with drugs. He knows this, and supports me. Since he did all this, knowing that he was in a bad position, is this a good enough reason to be mad/upset? I do feel this way because I feel he disrespected my position in his life, as a girlfriend. Is this a good enough reason to have feelings over? Is this a good enough reason to want to break up? 

To add she did not stay the night, he only asked her over to get pills. As he explained it. But I keep telling him I'm not stupid, and I know what to adults do alone. He says nothing happened. I trust him and I want to believe him, but I'm just so hurt. 

And to add again, they have been friends since teens. So about 15 years. We are both in our 30's. Ive never had to worry about a girl before. He treats me amazingly but sometimes has a hot head. But who doesnt? Im not worried he cheated, but more that he lied and tried to hide it. And completely disregarded how i would feel about his actions. I tell him all the time that he cant just think about himself anymore and that its an US. That a relationship we think of the other person. He does have alot of backround and hard past that we are trying to work through. So far its been really good but this ONE time thing. We have been together about 6 months.