OK two things.
First thing. Your relationship...
The fact that the photos were of local women, and that the person sending them was out with your partner at the time they were sent, would definitely raise my suspicions.
Without further information it would be extreme to say you should leave your partner, but I would be confronting him with this information and asking for an explanation. Obviously if he has form for this kind of behaviour you need to take this into consideration too.
Second thing. The photos...
As others have mentioned I would also be very concerned for the women. Presumably they don't know that naked photos of themselves are being circulated.
It could also get your partners friend, and your partner too if he sends the images on further, into serious trouble.
Since 2015 in England and Wales, it has been an offence for someone to share a private sexual image or video (online or offline) of someone without their consent and with the intention of causing them distress. Under Section 33 of the Criminal Justice and Court Act 2015, this offence is punishable by up to two years in prison.