Is this fair or selfish?

My partner and I are currently living with each of our parents. We are trying to buy our own place, but found out that we will be about £100 short each month.

My partner is desperate to move out into a three bedroom house as he has two kids who visit at weekends (when he split with his ex he had nowhere else to go and they were in debt so he had to move back home).

I am about to have a job change due to my health, meaning a £7K pay cut so if we want to be accepted for a mortgage we need to do it before I switch jobs. (We will be £100 short each month based on my new wages, we would have been fine on my current one)

So in order to make things work his Mother has offered to pay the £100 and my partner has accepted. To me this is scrounging and selfish but to him it’s his Mother being kind and supporting and it’s just what family does.

What are your opinions and why?