I think all this plotting to 'warn her off' looks a bit pathetic though. They were in public, you were there, nothing was happening, they were probably engaged in conversation.
If you know he's uninterested in her, and telling you she's a bit weird, what's the issue?
The main one is to increase his assertiveness and get him to simply take his hand away, or move away. If they are not good friends, this is not a hardship.
You don't need to stamp your territory. Over the years, I've seen women laugh a bit overly loudly at my husband's jokes, invite him and not me to coffee, be touchy feely, even engineer to go to events together away (he really was scared that time and didn't go!) He's also had some amazing women friends with whom he's laughing, chatty, has a fun time but doesn't place his hands on them or vice versa. Trust in him to handle it and don't go round warning people about this woman or holding her hand to make a point (you will look like the nutter, not her, she and certainly others will have completely forgotten about it).