The CSA payments cover his obligation to the children.
He can come and go to his home if he pleases, unless there is a court order to the contrary. Generally, divorcing couples will live separately, but some choose to remain in the same property until the divorce is finalised.
Assets such as the part share of the marital home will generally be divided once the divorce has been granted. This is when your friend and husband will decide how to split any equity / assets. Generally, the person wiy care of any school children will be allowed to remain at the property and can eother buy the pther person (husband) out and release his share of the equity...or the sale can be deferred until the child of school age leaves school, hen the property will need to be sold and the share divided at that point.
Your friend maybe paying the mortgage, but it seems like she also has exclusive use of the property, while her husband may be paying to live elsewhere.
This is simplified but there are variables depending on each situation.