Major boo boo on the numpty of a boss' part. The following is an extract from the employers' Q&A on the relevant law:
Can I dismiss a reservist?
No. The dismissal of a reservist before he/she is mobilised solely or mainly because you fear that he/she might be called up is a criminal offence. The same is true once the reservist has been mobilised.
Do I have to take back a demobilised employee?
Yes. You have a legal duty to allow the reservist to return to work following demobilisation, however long his/her absence on duty might have been. This requires you, within six months of the end of military service, to re-employ any reservist whom you employed in the four-week period before he/she was called up. Employment must be on the same terms and conditions as would have applied had the reservist not been called-up (e.g. including any pay rises awarded to comparable employees during the reservist absence on service).
If it is not reasonable and practicable to give the reservist his/her old job back on the same terms and conditions, you are required to offer him/her "the most favourable occupation and on the most favourable terms and conditions which are reasonable and practicable". It is not reasonable to give the reservist's job to his/her replacement, if that person has the same or less service than the reservist, or has had longer service but his/her employment was "of a kind that was... less permanent in character than the reservists" (e.g. if you recruited a temp to cover for the reservist, rather than simply moving another employee to his/her post).
Where a reservist is taken back in a new job, you must not dismiss him/her nor change his/her terms and conditions of service detrimentally for a period of up to 52 weeks (depending on his/her period of employment before mobilisation) unless it becomes unreasonable and impracticable to continue employment on the same terms.
If you fail to meet your obligations then, in addition to his/her usual contractual and statutory rights, the employee will have the right to apply to a Reinstatement Committee for an order or reinstatement and/or compensation. Failure to comply with a reinstatement order is a criminal offence.