Partnership cuts both ways.
He works away all week, then cares for you every weekend. Life is therefore difficult for BOTH of you.
You need so much support that people have to step in whilst he works, which means that you must need a lot of support on a weekend too.
Of course your husband should do that, but after he does that week in week out YOU should also be happy for him to have an occasional break.
I am a carer for my DH. I also work fulltime in a very stressful, challenging role. Without my occasional breaks (recommended by my gp and local Carer Support network) I honestly think I would have a breakdown. And then we'd be fucked. As, I suspect, would you.
Your husband is there for you every weekend. He now wants ONE weekend to see his friends. It is you who isn't being fair to your partner, not him.
You had weeks to sort out help so he could see his friend, but you thought he would and SHOULD change his plans. You're being selfish.