Yes, a trustee can defend a beneficiary’s trust contest, so long as they are acting in good faith. In fact, they may have a duty to defend the contest if the contest seeks to invalidate the trust as a whole or if the trust requires them to do so.
That said, trustees also have a duty of impartiality. Therefore, if two beneficiaries are opposing each other in a trust contest of an amendment that simply changes the beneficial interest of each beneficiary, it may be advisable for the trustee to abstain from participating, since their participation could be interpreted as them favoring one beneficiary over another.
A trust attorney can serve as a great resource for beneficiaries and trustees who wish to clarify the proper role a trustee should play in a beneficiary’s trust contest.