Let's take employee contributions first. In the tax year she leaves the UK, she'll have relevant UK earnings up to the point she leaves the country. This means she can pay tax relievable member contributions in the tax year up to the higher of those earnings and £3,600 (gross). For the next 5 tax years, she can continue to pay and get tax relief on contributions up to £3,600 a year. After that, no further tax relief will be given. Some providers will not accept any member contributions that aren't eligible for tax relief.
Employers get corporation tax relief by deducting their contributions from taxable profits. While Lexie's abroad, they'll continue to be able to do this so long as they can show that the contribution is still an expense for the company.