While I believe that it is irresponsible to leave a 15 year old on her own in a foreign country, I do think that she has paid the ultimate price for that.
You also have to think that if she had been 16, she could have gone to India on her own without her mothers consent. Maybe she could have sold a few pets!
However, the other thing that dismays me is that whilst Goa is probably statistically a very safe place, Anjuna is known for drugs and always has been since it started being a hippy/new age hang out back in th 60's.
I certainly wouldn't have a young girl unaccompanied in that type of area - unless of course that is what they were used to and that is why he saw it as ok - which to me is another story!
So to answer your question, yes I think that she should be investigated to ensure the ongoing welfare of her kids. However, in terms of risk assessment, you have to think that the risk of her doing something similar in the future must be close to nil going by the result of her last error of judgement. In the same way that I bet the McCanns will never ever take their eyes of their kids again.