We are not doctors here, but since hCG is a hormone, I would guess that it would not be a good idea for a breast cancer survivor to use this method for weight loss.  Your oncologist may be super-conservative about having his patients take other medicines/supplements while on Femara, but you would still need to inform him about any thing you take.  He can't make informed decisions about your health if he doesn't have all the information.  

I doubt that anyone has done a study to see if hCG increases cancer recurrence.  That would involve giving it to a group of cancer survivors and then monitoring them for recurrence compared to a control group. That kind of study would be expensive and potentially risky.  Someone might have done a study looking at people who had recurrences and asking them about hormones/supplements/vitamins they had taken.