I've just this minute bought a book for my Kobo using a Visa Debit card with no problem. All I can offer is what *should* happen...
When your friend clicks "Buy Now", she should see a screen like this (I've used the "Daily Deal" as an example & removed the last 4 numbers of my card after xxxx-) -
https://tinyurl.com/y8kq24z9 - then clicking "Complete Purchase" should finish the job.
If not, on the right in "Payment Details", you can see (it's small, sorry) the "Visa Debit" symbol - is that there? If not, click "Change" & review the card details. Make sure that here -
https://tinyurl.com/y8upex82 - you've clicked "Visa Credit & Debit".
I know this looks as though I'm stating what's obvious, but as I've not got a Credit card, I can't replicate the problem, sorry.
If you're still unable to complete the purchase, my next step would be contacting them directly. This link is for the "Contact Us" form -
https://tinyurl.com/y83p3jaj
you'll see at the bottom right - "Your contact options will appear once you complete this form" - so I'm assuming you'll then be given a choice of email address, phone no's, etc.
The Kobo is a nice reader & I hope your friend manages to sort through this & go on to enjoy her gift with no more problems :-)