Hi op.
Your friend is lucky to have you 
Depending in age of kids and financial situation here are some suggestions
Continue to invite fr weekends etc. also if your planning family day trip in holidays or a holiday that isn't already booked consider inviting them.
Offer to take pics of her with dcs, big bugbear of mine. All my pics are just dd.
Offer to babysit once a week/ month so she can join some kind of group or class and meet new people.
Offer to take all kids to local park for a couple of hours so she can get some housework done.
Pick her up a couple of bits with your weekly shop. Personal luxuries are usually the first cut back if your struggling. Don't offer, just do.
Cook extra and take round dinner to take some pressure off.
If she doesn't drive offer to pick her up when she's done her shopping or get big things like washing powder, sack of tatties for her.
Well done for making an effort and yy to making sure she knows you'll listen and still respect her if she whinges 