I've been through the family court system and it wasn't a nice experience one bit, but my ex was unilaterally trying to change the shared care arrangement our daughter was thriving under - she didn't want to or agree to attending Family Mediation. She applied to court for a sole res order, wasted THOUSANDS of pounds - probably about 8 thou, I self-represented myself , probably spending about 3-4 thou and got a shared residence order.
I recommend your friend joins Families Need Fathers.
By all means, go to three or four different solicitors and get a free initial 1/2 hour consultation and post back here what they say!!!
If you have information that is material to this case then you should immediately give a witness statement to him/make yourself available as you may be required to for a Finding of Fact hearing. You can be cross-examined at a FOF hearing and it's very serious.
But so is false accusations of domestic violence - which is what this is, and there are children involved, just think of that little girl growing up hearing that her dad was violent etc.
Don't believe the courts will 'sort it out', the family courts are civil courts and the burden of proof is the civil burden of proof i.e. on the balance of probablities - who tells the best story. If you have directly witnessed what you say you should be prepared to give a sworn statement.
Get your friend to join FNF and get him to go to a local FNF branch to get advice.
Your friend should be applying for a Shared Residence Order, this will ensure that he is a fully recognised equal legal parent (I have a SRO).
btw is your friend named as the father on the child's birth certificate? If so, and the child was born after dec 2003 then he automatically has Parental Responsibility.
Good luck to your friend, ask him to feel free to contact me and I can help out with practical advice and support. Also, he should get over to dads-uk forum which is a free forum for no-nonsense practical advice for dads.