Who was in the wrong in this case?

Now, I've known for a week that I was spending the day at her house today for mother's day. I was talking to my boyfriend about it last night, and he asked if he could come by for a little while and do something for her - he adores her and thinks of her as a second mother. I told him that since it was mother's day, she'd want it to be just her and her kids, and that it wasn't a good idea to ask her if he could come spend the day too. He pouted for a minute, then brightened back up, asking if I thought he could still do something small for her and come by only for a few minutes. I told him I didn't think that was a problem, as long as he stayed for no longer than about ten minutes. He happily agreed. This morning, while I was at mom's, he called me and eagerly began giving me details of a beautiful watch he wanted to buy her, wanting to know if I thought she'd like it (we've only been together for about seven months, so he still has to go by my judgement). I told him yes, but that he'd have to come by quickly because we had errands to run. He told me to go ahead and tell my mom he was coming, even though he wanted it to be a surprise, so we wouldn't leave and spoil everything. I did. She ended up getting super-pissed and yelling at me for telling him he could come. I told her it was only for about five or ten minutes, and she just said, "Whatever. It's no big deal." I tried to talk it out with her like we always do when we have disagreements and she would barely talk to me. My sister told me I was horrible. He heard it all and got really sad, because he'd only wanted to do something nice, and he could hear her acting so terribly. She acted sweet towards him when he got there. He gave her a big hug, said Happy mother's day, and gave him the watch, but then she randomly leaned in, hugged him, and said "Thanks for coming," basically telling him to leave, NOW. I walked him outside, and when we got to his truck, out of her sight, his head and shoulders drooped, and tears formed in his eyes, and he said, "She doesn't even want me here." It broke my heart. She KNOWS how easy it is to hurt his feelings and how much he loves her. Who was is the wrong? Should I really have not let my boyfriend come over for 5 minutes to give her a gift since she wanted it to be only us, or was she wrong for acting horribly to someone who was only trying to be thoughtful on her day?