Hi Michal,
Thanks for your interest in Syncfusion Products.
You can achieve the desired behavior by using InPlaceRichTextCell to display RTF content in the grid control. Please refer the below sample that works fine in editing and saving the changes to the RichText cells. To format the edited text in the RichText cells, right click on the cell to get a contextmenu and select font to set the formatting needed.
Here is a sample:
http://websamples.syncfusion.com/samples/Grid.Windows/I37249/main.htmPlease let me know if this helps.
Regards,
Subhasheela R