If its a work laptop it will be required to be locked. Generally that's company policy. Personal laptop, open to interpretation... My daughter and son both have laptops that auto-lock. They are adults and neither of them are in relationships and their education on computer safety from myself and the local high school really hammered home the necessity of locking your laptop.

If you are looking for proof of cheating, check his phone. That's where the apps will be and any communication apps (dm's through social media, snapchat, whatsapp, etc...)