This story is a bit long and rambling, but I would really appreciate anyone who takes the time to read it

My MIL really gets on my nerves!
So over 2 months ago my MIL mentioned going to a concert at Kenwood on 22nd August to my husband and me. At the time we told her we weren't sure I would be able to go but that my husband would at least go.
In the mean time, a friend of mine from the US made plans to come visit me and is flying in on the night of the concert. I didn't remember the concert since it was mentioned in passing over 2 months ago and I didin't know that she had bought tickets when I told my friend that she could stay with us that weekend and that we didn't have any plans. I checked with DH and he also thought we were free that weekend. My friend is coming in to Heathrow at 9:30 pm and I really want to be able to meet her at the airport to help her get to my place.
DH suddenly remembered the concert last weekend so called MIL on Saturday to tell her I couldn't go, BUT in trying to save face he said that my friend hadn't checked dates with us so we "didn't feel honour-bound to meet her at the airport" so I might still be able to go. Grr! As soon as he got off the phone I told him that wasn't exactly true and I DID feel that I should meet my friend. Then Monday night we were hanging out with his parents and his mom went off on me about the whole thing and told me that I HAVE TO GO to the concert and that I was being rude to not go and that the tickets were "quite expensive" (they are only about £30, so whatever). I told her who was coming because she had met her at the wedding and I was hoping it would make her more sympathetic and instead she said "Oh the Asian girl. She'll be fine."
I am really annoyed at how she spoke to me. She talked to me like a 5 year old. She kept saying that if I had prior plans with friends that evening I wouldn't break those plans to pick up my US friend so why would I break plans with my in-laws? And OF COURSE I would break plans with friends to meet my US friend! People are usually understanding that you would want to meet someone who was coming to visit you. My MIL compared the situation to that we are going to stay with his parents in Slough this weekend and they can't come to the train station to pick us up so we will take a short, cheap taxi ride to his parents house - where he grew up and has a key to get in. Not the same AT ALL if you ask me! And it won't be that easy for my friend to get to our place from Heathrow with the Victoria line being closed. The other thing that complicates it all is that even if I do go to the concert I probably won't make it back to our flat before my friend so my friend would be waiting outside at night with all her belongings in our sketchy neighborhood.
The only compromise I can think of is going to 1/2 of the concert and trying to bolt over to Heathrow to still meet my friend. (Which is really not ideal at all since the concert is in North London and Heathrow is South and it will take over an hour for me to get from the concert to the airport.) It seems like my MIL is personally insulted that I don't want to go to the concert and would rather pick up my friend, so I don't know if this will be an acceptable compromise. DH thinks I should tell my friend that she needs to stay in a hotel near Heathrow that night and I will meet her the next morning, but I don't like this idea at all! I'm so annoyed at the whole thing. I already didn't exactly like my MIL and now I really dislike her. I didn't want our relationship to be like this and don't want to strain things for the future by not going to the concert (she seems the type to hold a grudge), but feel like I can't leave my friend either. HELP! Any advice???