The way to compute a Green's function for a problem with reflection is described in [1].

You can download a notebook for computing other Green's functions with reflection from [2].

References

[1] A. Cabada and F. Adrián F. Tojo, "An Algebraic Method of Obtaining the Green's Function for Some Reducible Functional Differential Equations." arxiv.org/abs/1411.5507.

[2] F. Adrián F. Tojo, A. Cabada, J. A. Cid, and B. Máquez-Villamarín, "Green's Functions with Reflection" from Wolfram Library Archive—A Wolfram Web Resource. library.wolfram.com/infocenter/MathSource/9087.