Right, so based on this thread (and other similar ones), what can the partner do to reassure OP?
Tell the colleague to cut down on the messages - RED FLAG, shows he's feeling guilty
Not tell her to cut down on messages - RED FLAG, he wants to carry on messaging
Promise to stop talking to her - RED FLAG, he's only doing this to shut you up
Not promise to stop talking to her - RED FLAG, he's starting an EA
Give OP access to his phone - RED FLAG, it means he's just deleted everything dodgy, clear sign of guilt
Not give OP access to his phone - RED FLAG, what's he hiding on there?
Apologise - RED FLAG, it means he knows he's in the wrong
Not apologise - RED FLAG, he's a cheating bastard