Hello and welcome to the forum. I am sorry to hear about your relative's recent diagnosis. It is good news that the doctor feels that he was able to remove the polyps successfully. The BCG treatments are to discourage future recurrences.
I don't personally have any experience with BCG treatments, but there are many on here who have. What I do know is that doing the BCG regime has had very positive results for many of our members who have remained cancer free for multiple years, or if a recurrence occurred, it seems to have been readily dealt with.
In the meantime, if you search the forum for BCG, you will find several posts about it. The most often mentioned side effect seems to be fatigue for a few days afterward. There are some who could not tolerate the solution, and often it can be diluted to a weaker concentration still with good results.
I hope all goes well for her in the future.