I have the same sand in my tropical tank, but I really would avoid using it for my goldies myself. A lot of people seem to use it, though, with no ill effect.. and I just did some reading and it seems pretty divided. One issue is that it'll get stirred up a lot.. but this may be good, in reality, since it's really hard to put anything that stirs up substrate with goldfish. I'd personally worry mine would eat some of it and get impacted.. they are insane and eat everything!
I love my goldies but I always talk people out of it unless they are willing to start big. I didn't initially. I'm actually going to be building an indoor pond in the next year or two to house them.. It takes a lot of maintenance and while most people will religiously do what needs to be done for a while, a lot of people get really burnt out and the fish usually suffer for it. Personally it relaxes me, but I'm a weirdo.
There are a lot of really adorable fish out there that will thrive in 36 gallons and look wonderful, too! Just explore the options and look online, too! There may be fish that she will love that aren't in the bigger pet stores. If she sticks to her guns make sure you get a reallllly good filter and prepare to filter way over what they say gallon-wise. Also be sure to get a gravel vacuum and a bucket, or at least get a python to siphon. It'll be a weekly task for quite some time.
Keep us posted!