@Sajeagh123, I had hoped to hear more about their relationship and yours. However, based on the information you have provided, my take is:
He and OW have been building an emotionally intimate connection (at the least) for more than a year. You’ve seen two examples of the evolvement of their relationship, both involving his reaching out with gifts, which is atypical for him. I assume that there have been numerous interactions that have progressed their closeness during the past year.
The birthday roses were an affectionate statement, regardless of their cost. The thoughtful gift of the fossil and her delighted, heart-felt response speak to their mutual feelings of intimacy, validation, and reliance. He valued the object for OW specifically and she is deeply touched. He will be greatly gratified by both her appreciation and the photo of her holding his gift.
In my view he adores and cherishes this woman, and his feelings are not platonic. My H has female friends, but in no universe would he be reaching out with birthday roses or bringing special, romantic gifts from trips to anyone but me.
@Sajeagh123, your relationship is under threat. As I said in a previous post, I would investigate his devices. Knowledge is power, and any information you can gather will strengthen your position when you confront him. He is likely to gaslight/downplay/blame shift. I would have considered the roses to be a line crossed, but his manipulative wide-eyed ‘I just wanted to make my friend happy’ swayed you. He may try that tactic again to make you back off. Don’t.
I suggest that you both read Not Just Friends by Dr. Shirley Glass. She explains how a friendship can shift into EA territory when boundaries are blurred and emotional energy is channeled away from the primary relationship to the new one. He has already inflicted damage to your partnership via his inappropriate investment elsewhere.
I wouldn’t even consider moving forward with him until he gives you the full story, cuts contact with OW, provides open access to all devices, and works on his character flaws by plugging into IC and the survivinginfidelity site.