This is just my opinion, for a startup, if there is no possible way to advance the biz further for that day, as in, it's no way to make calls and brains are fried that is when life kicks in.
So by going with that logic, if you are in a meeting and he's texting his girlfriend, he's burning valuable time. Time is money.
Your time is just as valuable as an investors time as you are a key investor in the company. Would he text his girlfriend while the money guy is in the room? If so, he's in the honeymoon phase of his relationship and this will pass soon. If not, tell him your office hours are from 9 to 5 and he should be focused on the business at that time.
Personally, I'm a romantic and single so it's easy for me to say this. Because I know this about myself, I'm single because I'd rather succeed in my business as I've given up too much already to be where I am and unless I meet someone that makes me see God, farts skittles AND is able to guarantee my business succeeds while I have the lifestyle I want to live ---> business first!
Yesterday, a contractor I work with had to leave because of a date. We were in a meeting where things had to get flushed and final for the day. When he said he had a date at 7 and had to leave to shower, etc... I was happy for him. If it were my cofounder, I probably would have said, "Today isn't Friday and you seriously planned a date when we have all this work to do?"
Or, I would really help him achieve his work life balance by switching our hours to 3am to 12pm and having a no phone policy during those hours. This way, a bulk of his morning will be concentrated on work. In the afternoon you can focus on business calls and visits. That way, he will have all afternoon to text his girlfriend and all evening to spend with her. That is truly what work life balance would look like if it existed.
Good luck!