I gave my friend a loan and she hasn't paid it back?

I lent my friend some money 4 years ago and not had a penny back since. This summer their car broke down on their way to a cheap camping holiday in Devon. I paid for their car to be towed back to us so my husband could fix it - which he did out of his own pocket. I lent them my car so they could go on holiday. When they returned their car was mended. My husband doesn't know them as well as I do and thought we'd be paid back on their next pay day. Instead I was told they'd pay it 'in the next few months'. I know they're on an incredibly tight budget and they've both been ill as have their kids so I've been patient cos I know they'll pay it back. My husband thought I was being taken for a ride and told them so now they're upset, demanding apologies. They've paid for the clutch cable on principle but I'm stuck in the middle of an argument. My husband wants to protect my interests and my friend's wounded cos her character has been called into question. How do I sort this out?