No, your girlfriend is unlikely to face double taxation for this 3-month delegation to Belgium under a Dutch contract. The Netherlands-Belgium tax treaty and EU rules prevent double taxation for short-term cross-border work, as long as she does not spend more than 183 days in Belgium during any 12-month period. Since 3 months is well under that threshold, her salary remains taxable only in the Netherlands, provided her employer is Dutch and she does not have a permanent establishment in Belgium.
Here is the practical detail: The 183-day rule counts every day she physically works in Belgium, including commuting days. If the assignment extends beyond 3 months, the total days may approach or exceed 183, and Belgium could then claim the right to tax her income for those days. In that case, double taxation would be avoided through a foreign tax credit in the Netherlands, but she would need to file a Belgian non-resident tax return. Her employer's chaotic management is a red flag: they might not have arranged proper social security coverage. She should ask for an A1 certificate to confirm she remains under Dutch social security for the assignment. If they cannot provide one, she may need to register with the Belgian social security authorities temporarily.
Honest tradeoffs: The employer not paying for lunches or commuting expenses suggests they are treating this as a simple business trip, but the daily commute and potential extension make it more complex. She is likely entitled to some tax-free allowances for temporary assignments (e.g., for travel and meals), but only if her employer offers a formal delegation policy. Without that, she bears the costs and may miss out on tax benefits. Also, if she does not report her days or fails to get the A1 certificate, she could face issues with Belgian authorities later, especially if the assignment extends.
Concrete next steps: First, keep a simple log of each day she works in Belgium. Second, ask her employer's HR or payroll department for a written confirmation of the delegation and an A1 certificate. Third, contact the Dutch tax authorities (Belastingdienst) to request a certificate of residence for tax treaty purposes. Fourth, if the assignment exceeds 183 days, consult a cross-border tax advisor to file the necessary forms in Belgium. Finally, check if her employer offers a formal short-term assignment policy that includes expense reimbursements or allowances. These steps will protect her from surprises and ensure she only pays tax in one country.