$\mathrm{Amorphous} \mathrm{boron} \underset{△}{\overset{\mathrm{air}}{\to }} \mathrm{X} + \mathrm{Y}$; 

$\mathrm{X}\stackrel{\mathrm{hydrolysis}}{\to }\mathrm{Y} + {\mathrm{NH}}_3$

Incorrect about the product of the reaction '$\mathrm{Y}$' is

The reaction is given below:

$\mathrm{Amorphous} \mathrm{boron} \underset{△}{\overset{\mathrm{air}}{\to }} {\mathrm{B}}_2{\mathrm{O}}_3 + {\mathrm{H}}_3{\mathrm{BO}}_3$

${\mathrm{B}}_2{\mathrm{O}}_3\stackrel{\mathrm{hydrolysis}}{\to }{\mathrm{H}}_3{\mathrm{BO}}_3 + {\mathrm{NH}}_3$

${\mathrm{H}}_3{\mathrm{BO}}_3$ is boric acid and it is a lewis acid. It is shown below:

Since, it releases one ${\mathrm{H}}^{+}$ ions, it is lewis acid..