Suppose a UK citizen ("Alice") moves to the US for 3 years for work. I am aware that 1) when Alice stops being a UK resident, she is no longer able to pay into UK ISAs, and 2) when she becomes a US taxpayer, she will be taxed by the IRS on any interest/gains made on ISAs already held in the UK.
Now, the US has a $15,000 annual tax exclusion on gifts. On the other hand, suppose Alice has an adult sibling ("Bob") in the UK, who is not planning to hold any ISAs for the next 5 years or so. Alice sends Bob $15,000 (presumably tax-free, by the exclusion mentioned above) in her first tax year in the US, which Bob then invests in a stocks and shares ISA in the UK. On Alice's return to the UK after 3 years, Bob then liquidates the ISA with no tax implications from HMRC, and gifts the balance back to Alice, once she is no longer a US taxpayer.
Assuming neither Alice nor Bob die within the near future (so there are no inheritance tax implications), and ignoring any issues of trust, are there any legal implications for Alice and/or Bob? Additionally, are there any tax implications missed out in the argument above? Let's also assume that whilst working in the US, Alice personally holds no ISAs back in the UK.
This question was raised by a colleague; he thinks it sounds reasonable, I thought it sounded too sneaky to be a viable legal option. But I really don't know anything on the matter, so would be interested to hear from others. Thank you in advance!