Update to say thanks for all the advice- My friend's husband came round, started off taking the surface back a bit with fine wire wool and thinners but that didn't shift the dye at all, it had sunk farther into the wood, so he gently sanded the area and re-waxed it, looks lovely, but slightly less shiny than the rest of the table, so I'm going to wax the whole thing and that should give it a more uniform look. There are luckily a lot of small use marks on the table anyway, that I hadn't really noticed, dark rings, spots, that kind of thing. The patina is quite varied too , so a slight difference here and there won't matter. I am very relieved!
I'd spent the last two weeks telling her to always use a coaster as the tabel would mark with water . Grr. Am not so cross now though, the damage is done. She was tired out last night after the first day back at school. Anyway asked a friend to ask her husband who knows a lot about old wood, he thinks it will need taking back and re-waxing. The whole leaf, possibly the entire table. The top isn't pristine, there are scuffs and small marks, but nothing like this. It is an old oak country table so not highly polished.
To be fair on dd - she did mop the water up and may not have realised that dye was likely to come out of the card, or that it might leave water marks - especially if wooden furniture she normally comes into contact with has modern varnish and therefore the surface is more durable!
You can get "ring remover" from places like B and Q which may help with the water stain.
To get some of the colour off I would try rubbing the area with a damp, extremely well wrung out microfibre cloth, rinse out any removed dye from cloth and do the same again until no more dye comes out. Then immediately dry off with kitchen paper. It won't remove all but may make it a bit better and is unlikely to harm table.
I wouldn't attempt any sanding back yourself as it may result in a paler shade of wood if the piece is old and has developed a darker patina.
It is old, oak so waxed rather than french polished. I think it is probably of sentimental value as much as actual value, although it isn't a junk shop table more of an antique shop one. The property we are in is rented out as a holiday let (we are looking after all their animals, their own house is opposite). I will tell them, but wanted to have it looking as good as possible by the time they return. DH thinks we should sand it back and then re-wax it. The dye has really soaked in . I am so cross with dd. (dd2 has already broken a lampshade by charging about chasing, but it isn't old or expensive so we can replace that). Am slightly regretting saying we would house-sit now, the stress is terrible. I spent most of yesterday in a panic as the hunt was meeting at the top of the drive and I was worried the hounds would get in and kill something.
I'm with HDee - you need to speak to your friend about it if it is an expensive table and not start messing about with Brasso and stuff.
On a similar theme - my MIL used to have a table which was her pride and joy (not allowed to set anything on it, cleaned hourly that sort of thing). DFIL made the mistake of polishing it with furniture polish - leaving it all streaky. He consulted Mrs Beaton (or similar) which state he should use white spirit to remove polish. So he set about it with White spirits and took all the polish off the top. MIL was not pleased! Had to have it re french polished.
Irony is a few years later she donated it to SIL who has since ruined in.
I think you can remove water mark by carefully rubbing the area with Brasso and a soft cloth. Button polish to the nearest colour would bring back the shine If it is an antique table, the finish might be French polish and i would ask a French polisher to have a look.
Oh Blardy Hell. We are house sitting for a friend. They have lovely old furniture . DD spilt some water on a table last night, and although she tried to mop it up, she left the wet cloth on the table on top of some red card she'd been cutting out. Now the table is water marked, but worse, has red dye marks over it. Not a tiny patch either, about 6" square in splodges. I could cry. I bellowed at dd and she then howled all the way to school, so i now have parental guilt as well as a ruined table (she is 8, so old enough to be a bit more sensible). What can I do to salvage it ? Please don't say "nothing" <holds head in hands>.