No, Arran did not, as far as Roger was aware, have a girlfriend. But Lydia evidently got to the right conclusion before she had managed to finish her comment, which Roger was kind of glad for. Explaining that this had been more than a year ago wasn't something he particularly wanted to do. Instead, he was left to nod. "Yeah," he confirmed. His shagging of Cariad had effectively ended Arran's relationship with her, which was probably lucky because Roger really didn't care to entertain the idea of what would've happened if it hadn't. As much as he hated that him and Arran were no longer friends, he would have hated it a great deal more if Arran had stayed with Cariad after. He deserved better before and he certainly deserved better after.
Lydia was right in that Roger had never liked Cariad. Ever since Arran had first met her there had been something about the woman that Roger just didn't... like. She had a way about her that Arran seemingly found charming and Roger had found terribly off-putting. He'd said as much but Arran had never really seemed too bothered. Had always liked Cariad enough to ignore the fact that Roger disliked her. Roger had tried to like her, he really had, but it had never managed to go past just being polite enough to her to not look outright rude. So yeah, Roger could see why him having had sex with her seemed... odd. Lydia's words, asking whether he had a good reason for sleeping with Cariad startled a laugh out of Roger.
"Does anyone ever have a good reason for shagging their best mate's girlfriend?" He asked before downing the shot in front of him, making a face as he reached for another slice of lemon. "No, I don't imagine I did," he replied finally, chucking the now squeezed lemon slice on a plate, before refilling his shot but not taking it. The tequila was definitely making its way into Roger's system. "She was such a bitch, Lyds, he deserved better," Roger said really quite pathetically.