I have just emailed one of my flight logs to a friend who also has SD on an iPad so he can understand one of the flights I made recently.

He's just pinged me an email back asking how to load it an I have to admit I don't know.

I had assumed that you could 'click/hold' on the file in the email message and then use the 'open in' dialogue to load the file into SD, but nothing I have tried seems to allow access to the file. I then thought to perhaps 'drop' the file into the folder using iTunes but that doesn't work.

I can happily load the gpx file into SD Plan, but he wants to load it on his iPad.

Any advice please?

You actually can't do it. It's a future enhancement we're considering.
You can do it a slightly long-winded way by emailing it to him and him saving it to his PC, in the SkyDemon\Logs folder. Then he can inspect it on the PC, and also on his iPad if he saves his logs to the cloud.