I ignore obvious typos, but if I see someone really doesn't know how to spell something, I mention it in passing. It's up to them - they can choose to learn, or they can ignore it.
A lot of people on here, especially on this section, are speakers of other languages, trying to improve their English. We do them no favours if our own grammar and spelling are sloppy.
It's no harder to type 'their' than 'there'. Why not just get it right?