The best and easiest way to complete a transfer from your EUR sync. account to a GBP bank account is by exchanging the currency from EUR to GBP within your sync. app. You will then be able to transfer the funds to your friend's bank account using the Faster Payment System.
Alternatively, you may transfer money directly from your EUR account to your friend's GBP account, however, transfer fees may apply. Please refer to the in-app transfer page for applicable prices.