Cuppa your friend is vile. And she's wrong about it's less chance of having 15 MCs in a row vs a 1 in 48 chance of downs. I assume she meant there's more chance of your baby to be that 1 in 48 then having that many MCs in a row. Clearly, if there's no underlying issues, then 15 MCs in a row is very very rare. And that she's probably right if you are that very unlucky one. However, it's much more likely that you do have an underlying cause of all the MCs. It could be undiagnosed but still there is probably an underlying cause.
There are two numbers for that 1 in 48 chance. Firstly, unless you are an older mum or have a family history for trisomy abnormalities, then you are plain unlucky to have hit the jackpot for having a screening outcome below the amnio threshold, which is 1 in 150. The following NHS page gives the probability for different age groups
www.nhs.uk/Conditions/Downs-syndrome/Pages/Causes.aspx
I'm 39 and for a 40 year old mum the chance is 1 in 100. Which is actually below the threshold of 1 in 150. So therefore my chances of getting a risk below the threshold would be much higher, and I think you are relatively young isn't it?
Secondly, even at a risk of 1 in 48, you are 98% likely to be carrying a perfectly normal child.
Is your friend a scientist? I don't think she understands statistics at all.