Leave her to sleep. Might mean she doesn't nap again today and you might have to put her down to bed earlier than normal. The earliest I have ever but DS to be was 6.20pm! It didn't affect his morning waking time.
I also think 6mo is usually around the time when Babies start to drop that later afternoon nap am they are able to stay awake longer, usually having one nap in the morning and one long one after lunch. So you might find your little ones routine changing a fair bit in the next few weeks anyway.
My 6 month dd usually has a pretty good routin of feeding every 3 hours (plus breakfast and supper) and sleeps roughly 1 hour in the morning, 1 and a half hours early afternoon, 45 mins-1 hour afternoon and 10-12 hours overnight. Last night she was up most of the night teething and I finally managed to get her to settle for a rest earlier, she missed her morning sleep as was too overtired to settle, and now she's been sleeping for 2 and a half(!!!!!) but she's due for a feed... I tried making a bit of noise in the room to see if she would wake up but she's really deeply asleep. Should I leave her to wake up in her own time or will that totally mess up her routine and make her not sleep well again at night?