Would my friend be sinning if she were to remove the niqab and wear the hijab that reveals her face, due to the difficulty of going out with it in an infidel country?

A woman's covering her face from non-mahram men is obligatory according to the most preponderant opinion among scholars and the majority of scholars, especially when there is fear of fitna (temptation/strife) and the spread of corruption. If a woman adheres to the niqab, she has done well, and she should not abandon it for a less weighty opinion. There is no real difficulty preventing the wearing of the niqab abroad; the difficulty mentioned in the question is an imagined matter on which no Shar'i (Islamic legal) ruling can be based unless actual harm is confirmed. Westerners do not differentiate between the niqab, the khimar, or even head covering; all of these are considered strange to them. A Muslim woman should be patient, steadfast, cling to her religion, and strive to earn the pleasure of her Lord.