lol at dirty and smelly. lol at neglected!
My daughter has had horses since she was two years old. It's not a hobby, it's her life. She met most of her lovely friends through horses, and her boyfriend, and our lives revolve around horses. I even make our living (such as it is!) through them now.
My son also has all-consuming 'hobbies' - football and cricket. It's not just horses that consume teenagers and young adults.
I have encouraged this from them both being very small. Both DC are now grown up and they are excellent at time-management, fit and healthy, have loads of friends, are both studying for degrees and both work.
One of my DD's part time jobs (she has several) is riding out for a professional yard. She also has her HGV licence and has achieved all sorts of other things that she would never have done if she wasn't into all things equine.
We also have dogs as most horsey people do, and the house is a bit of a tip, and I broke the iron about a year ago and haven't missed the lack if it!
My exH has been to watch my DD the grand total of twice in her whole life when she has been competing. He calls her once in a blue moon, never encouraged her when we were married, in fact was hostile to the whole thing. Now he can't understand why she can't be bothered with him.
Your H needs to put masses of effort in to get back into his DD's life. If you feel she is not up to standard as a 'young lady' re the smelly thing she will have picked up on this. She doesn't need 'other interests' as horses give you many, many different skills for life. Your Dh really shouldn't be giving up with the contact. Quite the opposite. By the sound of it he has a lot to make up for. Does the new chap in her mother's life fit in around the horses? I bet he does.