MG with another guy is a 100% red flag for DPC, but someone from SG is probably. That girl for whom the audience doesn't have strong feelings. Definitely not Arieth, almost no one wants to get a crabjob. Lily? She lives with Elena, they are on good terms, but many people like Lily. A riot is possible. Sarah and Melanie are good hypothetical candidates, but they don't seem to be very close to Elena.
Due to the fact that the audience does not want to share a girl with someone else, I came up with a foursome idea where they don't swing partners and just watch.