חטא בשביל שיזכה חבירך
Are you allowed to sin to save your friend from a worse sin? The gemara in Shabbos (4a), tommorrow's daf, discusses this point. The gemara discusses the following case. Someone put bread in the oven on Shabbos. If it stays in the oven they will violate the issur d'oraysa of bishul. Taking it out would violate an issur d'rabbanan of r'diyas hapas. Are you allowed to take out the bread and save your friend from an issur d'oraysa? The gemara says no. Tosfaos asks, but we find cases where people freed their slaves (which is an issur d'oraysa) so that they could do a mitzva? Tosafos gives 2 answers.
1. A mitzva rabah is mutar. You are allowed to do an aveira to allow your friend to do a mitzva rabba
2. It is mutar if your friend is not at fault.
This is very relevant halacha l'maase regarding kiruv. Many times people working in kiruv need to violate an issur d'rabbanan, based on these 2 opinions in Tosafos there is room to be lenient. This is discussed in earlier poskim regarding cases where a person's child was kidnapped on shabbos to became a priest or nun. Can the parents be mechalel shabbos to try to save them?
These are very difficult questions and need to be answered by a real posek. I am posting this to raise awareness of the issue and show how while learning daf yomi you get many yedios in shas.