US Withholding tax

I have a UK based client that receives dividends on US shares, he's non-resident and has completed form W-8BEN so has only 15% tax deducted at source by US. He is basic rate tax payer and I'm wondering if/how he can reclaim the 5% difference in tax (eg US have deducted 15% but he is liable to just 10% UK tax).

First client I've come cross like this, advice appreciated.