Its not extra relief at all. You are bringing forward next years contributions but you will only be eligible to claim the tax relief in the following year. If you can contribute 10k in year 1 and year 2, claiming the tax relief in each year. Instead contribute 20k in year 1, and claim the tax relief in years 1 and 2 as normal. You are carrying forward the proportion of your contribution that has not been grated tax relief, to the next tax year.
The advantages of this are that your pension investment benefits from the tax sheltering for a longer period of time than would otherwise be the case. What you are trying to capitalise on is time in the market, rather than drip feeding your investment. The risks are that your contributions are being made in the expectation that you will be able to claim the tax relief in the future i.e. your circumstances change such that you cannot claim 40% tax relief, the opportunity costs of an alternative investment, other risks such as needing the cash for other uses, change in the tax environment etc.
Just to be clear, I am advocating for claiming the tax relief in future years, rather than not claiming it at all.
I would have thought that these circumstances are ideal for pre-funding, given the 10 year timeframe to retirement, substantial (cash) assets, no other liabilities or discretionary requirements for the money, a specific investment objective of maximising wealth for retirement - and the pension is the only game in town from an investment perspective given the overall circumstances.
Regarding the risk of claiming future tax relief, its 'steady as she goes' for their financial circumstances over the next while. The intention is to maximise the tax relief anyway. It'll have no effect on their lifestyle in any case. Make the contributions from their cash savings. They stop making AVCs (monthly cash flow increases) and in an emergency they have recourse to the 50k held for their child (or in extremis would be able to take a loan which could be paid off by the 25% lump sum down the road).