Instead of owning, have you considered a full-lease? That way if you choose you do not want the horse you can simply return it to its owner, instead of buying/selling in this economy.
-Since you say it is 'a very nice barn' full board will an absolute minimum of $300+ per month. Usually a nice barn, good services, and good facilities = more money. Expect to pay $400+ per month.
-Lessons can range from $40-80 per hour. Hay should be included in the board, however if the horse requires supplements or certain feed, that is another expense. Could be $10 a week, or $50. Depends on brand, feed type and country.
-Farrier, that's roughly $70 every 6-8 weeks, however if the horse is shod, then that will set you back $150.
-Covers, tack, general supplies add up as well. Buy decent tack and covers otherwise you will need to have them replaced within a year of purchase. The horse will need shots once a year (twice depending on your area and situation) which costs $100+ depending on the vet and shots.
Per year:
Board - $3600 (at $300 a month) - $4800 (at $400 a month)
Lessons - $3120 (1 lesson a week at $60)
Farrier - $600 unshod - $1000 shod (both minimum)
Shots - $100 minimum
Feed - $500 (varies significantly)
Supplies - $200
Extras - $1000 (new tack, covers etc)
TOTAL = $9120 - $10720
Expect to pay above $10000 as i gave the minimum cost values.
Always have a buffer of at least $1000 in the bank which is reserved for emergencies. The horse may become ill, which will use up that pretty darn quickly.
If the mum can afford it all and you have help at the barn, then go ahead. As long as you have experienced people at hand then you should be fine. I would however recommend a lease as opposed to buying. At least do that to ensure you won't tire of the horse like i've seen many 'eager' people to do. They take a lot of time, money and effort. You may decide after a few months that you don't like the horse and in this economy you will struggle to sell it. If you do, it will be for much less than you bought it for.