Sorry, I don't think she should be "scared" but be informed... There is a difference.
She, they need to understand that finding a animal is the easy part.
It is the dedication of daily care the animal requires to keep it a nice animal.
Besides the initial expense of acquiring tack and grooming supplies we all know it is the commitment of time, and much energy that is what fails most who take on a horse unprepared.
It is then also the continual drain to their finances of many hundreds of dollars a month and could rapidly go to thousands for one illness taking place.
"They" are not going to listen to your words of wisdom, they have already proved that with not listening but telling you about horse care and such, and for that reason...you are best to state the cold-hard facts and walk-away.
be sucked back in to their problem now in the making or it will
become your problem.
Tough love in this case is
going to create a rift...
How big a rift, and if you want the Grand Canyon or a small earth split from dry weather is something only you can decide and will need to live with.
This is still your husbands family and he will
feel the pain.
The fact they come to your home, you've offered her your horse to ride tells me you are closer than most in-laws situations.
There is a middle road here too...
One I think being clear of expenses they will face and the fact you can not
step in and do more for them is paramount they get it...you can not and will not bail them out when they are in over their head.
That is your biggest concern, being in over their head financially and her being over-faced with inexperience being hurt.
Most people love the romantic idea of horses and riding...never realizing how much work, expense and changing of your lifestyle needs done to accommodate that animal now in the family so they receive proper care with all areas falling under "care"...and how much that $$$ costs every single month.
But "scare her" might just be the catalyst to backfire & where they wobbled it will now be "I'll show her..."
Think this through very carefully before going one-step further in any direction.
It is easy for all of us to be chair-side therapists and advice giver.
It is far harder to execute what needs done and live with the consequences when it is "family".
I wish you good luck...no matter what you do you are being "the bad-guy"...