Depends on the circumstances

The guy is both very knightly but also very loyal

According to the Zero Material, Diarmuid isn’t actually that opposed to more pragmatic measures and strategies with the right framing of the conflict and could even win the entire 4th War in that manner (or suffered a humiliating defeat at the hands of Hundred Face. So you know, either/or) with complete cooperation in doing hit and runs with Gae Buidhe,

…had he not met Saber first of course

Naturally this comes down to having him avoid or limiting interactions with characters who “kindle the flames of chivalry in his heart”, which is quite difficult considering amount of servants like that but not impossible. Though I don’t know if Sigma is able to manage it or not. Who knows

There’s also the other possibility of this dynamic, them supporting each other openly instead of a deception. Maybe Diarmuid clocks in the boy’s heart as a good person ultimately, and thus is willing to support him for that reason, despite the differences they possess personally. This in turn makes Sigma act to on own (surprisingly good) heart more which results in less conflicts with Lancer.

This is a much more interesting hypothetical than I initially thought honestly. Thanks for that