My boys are 18 months and 4 days apart, I fed thru pregnancy and am still tandem feeding now. Yes milk supply drops and DS1 didn't feed for as long but once DS2 was born and supply came back they were both feeding with no issues.

All she can do is keep offering the breast some children will naturally wean themselves as taste changes along with supply but she can't make her other child keep feeding if she wants to stop herself.