No ,not if the 'Ex' has custody. I am assuming you mean to remove her permanently from the 'Ex'. The new partner has nothing to do with it, drugs or not. Custody of the child is a matter for the family court if the two parents can't work out a sensible arrangement between them.
As you say they only split up a few days ago I assume the question of custody has not been settled yet? Until custody is settled the child would normally stay with the mother. Also as they only split up a few days ago they they may get back together, so it is best to wait a while and see what happens.
It is very much in their interest to sort things out between them. If solicitors and the court get involved ,things get VERY expensive VERY quickly.