OK, with the mix of serious and joking posts, I'm not sure where to go, but I'll take the serious route, and I hate to do this as a new member of the forum, but I'll try to be brutally honest.
As a guy with a bit more experience (41) I say forget this woman if you haven't already. Don't text her, don't return the umbrella (unless she specifically asks for it), and don't answer any texts from her (short of a request for her umbrella). If she wants her umbrella, arrange to meet to give it to her, don't act hurt, angry, or emotional at all, just give it to her and leave. Keep any conversation as businesslike as possible and limit it to about 5 minutes or less. Have a spine, have some self-respect, you deserve better (I'm new here, so I guess I'm just assuming, but damn, even Bernie Madoff deserves better than that witch).
Here is the bottom line deal, she was way out of line, and to an unforgivable degree. Unlike what some posters have said, you aren't in friend territory, she is just taking advantage of you (and don't let it happen). I have never done to a woman what she did to you, I'm too nice a guy, but I have friends (men and women) who have done exactly that sort of thing to someone. I can tell you with zero doubt that she has little to no respect for you, and at best you might get one or two nights from her and no more (and probably not even that).
I suppose dating more than one person, at least until something is serious and expressly exclusive, is pretty common, especially among older single people. However, if she has been dating him off and on for a while (am I remembering right that it was over a year), she shouldn't be dating anyone else (not cool to him or you)- the only acceptable thing here is for her to break up with him and date others or to stay with him and only him. A year is exclusive territory, if not you have someone who is a cheater (could they be serious and you are the "other man"), a user, a liar, a woman who has trouble committing, or any combination thereof.
Now, lets assume they were broken up for a while and only recently got back together so being in dating others territory is acceptable, she was still well out of line. She was with you. You took her out, spent money on her, and the two of you were enjoying each others company. Meanwhile this wasn't a one time hookup, you are just starting to date, apparently with the goal (possibly mutual) of a relationship. Where in there is it acceptable to kick you out, presumably to sleep with someone else? I'm assuming at the moment that he knows she is seeing other people and she isn't cheating (see prior paragraph), she should have told him that she can't see him because she is on a date with someone else. Full stop, nothing more to say, and you should have specifically asked her to text that to him. If she can't say that then he probably thinks they are exclusive and she is cheating with you (once a cheater, always a cheater, I'd never trust a woman that cheats with me on someone else). The fact that she asked you to leave is highly disrespectful and outright nasty to you. There is an exception, a true emergency. If he was in an accident, needed bail, was in the hospital, etc. then that might be an acceptable reason for you to leave so she could tend to him, but then if that is the case, she could/should have told you so.
Kick her to the curb, she doesn't even deserve an explanation.